Field of the Invention
The present invention relates to loudspeaker systems generally, and more particularly to a subwoofer system for producing frequencies in the low or bass range of the audio frequency spectrum.
Subwoofer systems designed to produce frequencies in the lower bass frequency range (generally below 200 Hertz) typically employ a bass reflex enclosure system using a single transducer (sometimes herein referred to as "drivers" or "cone drivers") placed in a large chamber having a port to tune the system to a desired low frequency. The port causes the back wave of the speaker to become in phase with the front or direct wave, thereby adding to the acoustical output of the system. In order to gain power from a bass reflex system, the system must be designed so that it is a high "Q" system. The disadvantages of high "Q" systems, however, is that they are generally effective over no more than one octave.
It is also known to provide a double tuned bass reflex enclosure in which the enclosure is configured with two tuned chambers in front and/or back of the speaker and in which each chamber is tuned to a different frequency. This and other similar systems using one driver (or two drivers in a push-pull configuration) to cover several octaves have the disadvantage of being inefficient and impractical in environments where large acoustical outputs are required. Where maximum efficiency is desired, an enclosure can only be optimally tuned for operation over one octave, even if the bass reflex enclosure is tuned to more than one frequency, i.e., is relatively broadband. This can be understood by the fact that the acoustical output of a driver is determined by the size of the driver's piston or cone and the cone's maximum excursion. At low frequencies the reflex enclosure behind the driver needs to have a large volume and needs a large diameter cone to move a large volume of air in the enclosure. At higher frequencies the cone and chamber need to be smaller in order to obtain high output. Using a large cone with a small tuned chamber to produce higher frequencies, or a small cone with a large tuned chamber to produce lower frequencies is inefficient.
Since bass reflex speaker systems have heretofore generally been limited to either producing one frequency at a time ("boom boxes") or to inefficient broader band systems, high output power systems capable of reproducing complex sounds, such as music, without distortion have not generally been available. For example, where a single driver is used with a broadband bass reflex design, the speaker, when driven with music, is generally capable of producing less than 0.5 acoustical watts, which may be appropriate for a home hi-fi system, but not for a speaker used in most commercial applications.
A related problem with conventional designed bass reflex systems is that, when the driver is forced to follow the complex music signal, the excursion of the driver cone, and hence the acoustical output of the driver, is limited. Conversely, when the speaker is driven to higher output powers at a single frequency, the driver will have no excursion left to produce other frequencies, meaning it cannot accurately reproduce a complex low frequency sound. For example, one 15-inch diameter driver can produce one acoustical watt at 50 Hertz with a maximum excursion of one inch. When the driver is placed in a sealed chamber, the back wave is lost, but when it is optimally tuned (a Heimholtz resonator) to 50 Hertz the speaker can produce up to two acoustical watts by utilizing the back wave. When operating at this full power, there is no advantage to any other tuned chamber associated with the driver because there is no excursion left in the driver to drive any other frequency.
The present invention overcomes the above limitations and trade-offs of conventional bass reflex speaker systems by providing an improved bass reflex speaker system capable of efficiently producing bass frequencies at high acoustical power levels, and to do so over substantially the entire bass region of the audio frequency spectrum, i.e., in the audio spectrum generally below 200 Hertz. Using a bass reflex speaker system in accordance with the invention, complex low frequency music signals can be accurately reproduced (i.e., without distortion) at high sound pressure levels suitable to commercial applications.